The present invention relates to the field of tape recording medium longitudinal position sensing.
Tape recording devices have a recording medium wound around one or two spools inside a housing. The medium is moved at a low speed during read and write operations, and at a higher speed during search and rewind operations. A common mechanism for estimating the longitudinal position of the medium with respect to a read-write head is a tachometer coupled to one of the spools. Linear tape movement is translated into rotational motion by the spool. The rotational motion is translated into electrical pulses by the tachometer. Counting the electrical pulses provides an approximate relative distance traveled by the medium. When referenced to one end of the tape, the relative distance traveled becomes an absolute longitudinal position.
Knowing the absolute longitudinal position of the medium is useful when searching for specific data stored at a known position. A typical search operation involves two phases. First, the medium is accelerated to the search speed to move the desired position on the medium near the read-write heads. While the medium is moving at the higher search speed, the tachometer""s electrical pulses are counted to estimate the medium""s longitudinal position. The second phase begins as the desired position draws near the read-write heads and the medium is slowed to the lower read-write speed. The second phase of the search operation is conducted at the low speed by reading information from the medium until the desired data is found.
The time required to complete the search operation is determined in part by the accuracy in the estimated longitudinal position of the medium. Greater estimated position accuracy during the higher search speed allows the desired location on the medium to be positioned closer to the read-write heads without overshooting. Closer positioning at high search speed results in less search time at the lower read-write speed.
One approach to improve the estimated position accuracy has been to incorporate pre-written physical positions in the medium. Typically these physical positions are written in the servo tracks in such a way that they can be read while the medium is moving at the high search speed. The numeric range and physical spacing of the physical positions are selected so that on any given tape recording device each physical position is unique. Knowing the longitudinal position of the desired data in terms of the nearest physical position allows for fast search operations.
The physical positions are normally written into the medium before the medium is cut to the proper length, wound around the spools, and installed in the housing. This causes the first physical position in each medium to have a random value. It also causes the physical positions in some tape recording devices to rollover from the highest value to the lowest value in adjacent positions. As a result, the tape drive cannot treat the physical positions as an indication of an absolute position from the end of the medium For instance, a physical position having a value of zero may be near the start of the medium, near the end of the medium, or not in the medium at all.
Randomness in the initial physical position is easily compensated for with end loaded media. The first physical position read as the medium is loaded can be used as a reference point from which all relative, or logical positions on the medium can be determined. The first physical position may be added to all relative or logical positions to convert the relative or logical positions to the actual physical positions. Alternatively, the first physical position may be subtracted from all other physical positions to convert the physical positions to the relative or logical positions.
This approach of using the first physical position read after loading the medium as a reference point does not apply when the media is first loaded at the center, or at another non-end position. The tape drive has no quick means to determine if the first physical position read after a load is truly at the physical center of the medium or not. Tolerance in the servo system of any given drive, variations from drive to drive, and random unloads can cause the medium to load at different position at each independent load operation. The tape drive could move to one end of the medium immediately after loading to read the physical position nearest that end, but doing so defeats the purpose of loading the medium at the center.
Variations in the medium load position can cause several different problems. First, the medium is usually formatted immediately after the first load. Formatting assumes that the medium was loaded at or near the true physical center. When this assumption is false then an Media Information Region (MIR)(which contains administrative data) will be formatted off-center and the medium to either side of the MIR will be of different lengths. When writing to the short side of the MIR, the physical end of the medium may be reached unexpectedly. When writing to the long side of the MIR, part of the medium may remain unused. Other problems surface on subsequent loads if the medium is loaded at different initial longitudinal positions. For example, the MIR may be missed entirely, causing the tape recording device to be interpreted as a blank tape. This may make the tape look like a scratch tape to the host system and customer data could be lost. Another problem is that data written near one end of the medium after an earlier load may be beyond the servo limits computed from the present load position. The data is still in the medium and thus can be recovered, however, performance will suffer for those tape recording devices that are loaded off-center.
In center load situations, the tape drive must assume that the medium is loaded at or very near to the center. Once loaded, an initial scan of the medium around the load position can be made to find the MIR or another recognizable information that can be used to establish a reference position. If nothing is found by the scan, either an assumption is made that the medium is blank, or time must be spent searching for one physical end of the medium.
The present invention is an improved tape storage device and methods of using the tape storage device. Existing tape storage devices have pre-written physical positions stored in the longitudinal direction of the medium. Physical positions are pre-written in the medium before the medium is cut and spooled, so the first physical position in any given tape storage device has a random value. The improvement is storing offset information in one or more locations in the tape storage device to enable a tape drive to convert the physical positions into logical positions without the need to find one physical end of the medium. Optionally, the largest value of the physical positions (called a maximum physical value) may also be stored in the tape storage device. The maximum physical value supports the physical position to logical position conversion when the physical position values undergo a rollover in mid-tape. The tape storage device may hold the conversion information in the medium, or in a nonvolatile memory embedded inside a housing that surrounds the medium.
A method of determining and storing the offset and maximum physical value information in the tape storage device is also provided by the present invention. An offset is determined by an initial physical position read immediately after the medium is loaded in the tape drive. The initial physical position will be either the physical position nearest one end of the medium, or a central physical position. To determine the maximum physical value, all of the physical positions are read and the largest value remembered. Once the conversion information has been determined, it is written into one or more locations in the medium, or into the nonvolatile memory.
The present invention includes methods of using the offset and maximum physical value information. After loading, the offset is read from the tape storage device. The maximum physical value may be a constant known to the tape drive if all compatible tape storage devices use the same physical position range of values. Alternatively, the maximum physical value may be read from the tape storage device itself, allowing different lengths of medium to use different ranges of physical positions.
All conversion methods start with the basic process that the logical positions are the physical positions minus the offset. Variations in the conversion methods are described to account for a rollover condition where the values of adjacent physical positions step between the maximum physical value and the smallest physical value. One method of rollover detection checks the change in the physical position values between the current physical position and a prior physical position. A second method performs the basic physical position to logical position conversion and then checks to see if the resulting logical positions fall between set boundaries. A third method creates a lookup table that provides a one-to-one relationship between the physical positions and logical positions.
An advantage of the improved tape storage device and associated methods is that they allow subsequent loads of the medium at random longitudinal positions. The tape drive does not have to find one physical end of the medium to establish synchronization between the physical positions and logical positions. Conversion from the physical positions to the logical positions can begin with the initial physical position read anywhere along the longitudinal length of the medium. This ability allows center loaded tape storage devices to maintain their fast initial search capability as compared to end loaded tape storage devices.
Accordingly, it is an object of the present invention to provide an improved tape storage device that stores conversion information that enables a tape drive to convert physical positions pre-written into the medium into logical positions.
Another object of the present invention is to provide a method of determining and storing the conversion information in the tape storage device.
Yet another object of the present invention is to provide a method of converting the physical positions read from the medium to logical positions, taking into account the possibility of a rollover of the physical positions at any point along the longitudinal length of the medium.
These and other objects, features and advantages will be readily apparent upon consideration of the following detailed description in conjunction with the accompanying drawings.